feat: empty CRON_SCHEDULE keeps container alive for manual docker exec

Three container lifetime modes via CRON_SCHEDULE:
  unset          — run once on startup, exit
  empty string   — sleep infinity; use docker exec -it winnow winnow
  cron expression — run on startup, then on schedule

This replaces the need for a separate MANUAL_MODE env var. The empty
string is a natural "I want the container alive but unscheduled" signal.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

entrypoint.sh

@@ -2,11 +2,19 @@
 set -e
 export PYTHONUNBUFFERED=1
 
-# 1. Run the job immediately on startup
+# CRON_SCHEDULE controls container lifetime:
+#   unset          — run once and exit
+#   empty string   — stay alive, run nothing (use: docker exec -it winnow winnow)
+#   cron expression — run immediately, then on schedule
+
+if [ "${CRON_SCHEDULE+isset}" = "isset" ] && [ -z "$CRON_SCHEDULE" ]; then
+    echo "▶ CRON_SCHEDULE is empty — manual mode. Use 'docker exec -it winnow winnow' to run."
+    exec sleep infinity
+fi
+
 echo "▶ Running on startup..."
 /app/.venv/bin/winnow
 
-# 2. If a schedule exists, start the scheduler
 if [ -n "${CRON_SCHEDULE:-}" ]; then
     echo "▶ CRON_SCHEDULE set to: $CRON_SCHEDULE"
     echo "▶ Switching to scheduled mode..."
@@ -14,4 +22,3 @@ if [ -n "${CRON_SCHEDULE:-}" ]; then
 else
     echo "▶ No schedule set, exiting."
 fi
-